Turbochargers for gasoline and diesel internal combustion engines are devices known in the art that are used for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine to spin within the housing.
The exhaust gas-driven turbine is mounted onto one end of a shaft that is common to a radial air compressor impeller mounted onto an opposite end of that shaft. Thus, rotary action of the turbine also causes the air compressor impeller to spin within a compressor housing of the turbocharger that is separate from the exhaust housing. The spinning action of the air compressor impeller causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before the air is mixed with fuel and combusted within the engine combustion chamber.
The turbocharger turbine section comprises a turbine wheel, a wheel heat shroud, and a turbine housing. The turbine exhaust gas inlet is positioned along an outer diameter portion of the turbine housing. The turbine is a centripetal radial, or mixed inflow device in that exhaust gas flows inward, past the wheel blades, and exits at the center of the housing's diameter. Expanded engine exhaust gas is directed through the engine exhaust gas manifold into the turbine housing. The exhaust gas pressure and the heat energy extracted from the gas causes the turbine wheel and attached shaft to rotate, which drives the compressor impeller.
In a traditional turbocharger, the turbocharger compressor section comprises a compressor impeller, a backplate, and a compressor housing. The compressor air inlet is positioned at the center of the compressor housing diameter. It is a centrifugal, or radial-outflow device in that the air flows outward, past the impeller blades, and exits at the outer diameter of the housing. The rotating compressor wheel draws ambient air through the engine's air filtration system. The blades of the impeller accelerate, compress and expel the air into the compressor housing where it is directed through ducting to the engine intake manifold.
A central housing and rotating assembly supports the common compressor impeller and the turbine wheel shaft in a bearing assembly or system. Seals separate the center housing from both the turbine and compressor housings.
Conventional turbochargers comprise a single compressor impeller disposed within the compressor housing. Such single-stage compressors are known to provide a fixed pressure ratio at operating conditions that may not produce a desired level of pressurized air or boost at high RPM engine conditions.
It is, therefore, desired that a turbocharger be constructed in a manner that provides a higher pressure ratio, than conventional single-stage compressor turbochargers, at operating conditions where it is desired to provide an amount of pressurized air at high RPM engine conditions, thereby expanding the practical operating window of the turbocharger.